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Old 10-03-2017, 11:04 AM   #1
satsrbz
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Default Tach (not engine) goes to 8000 rpm.Back to zero on restart

1999 SL2. 50,000 miles.

Driving the other day, everything fine then tach started going higher and higher, ending beyond 8000 rpm.
No service lights. Car ran fine.
Tach stayed beyond 8000 when I shut engine off.

After I finished errand, I restarted car and tach moved back to 0 and went to correct rpm over about 5 second period.
Video attached of return to zero on start-up. I guess not. Can't upload video.

It is intermittent. Sometimes days go by with no problem.
If I try to restart shortly after it goes to 8000, it doesn't reset to zero.
It needs to sit for some period of time and haven't figured what the threshold is for that yet.

Was able to watch it climb last night. As I accelerated the tach would go up, but not down, as rpm changed and transmission shifted. It would go up a few hundred rpm on upshift or downshift, or change in rpm, until it maxed out over 8000. Again, the engine ran normally, no over revving.

I don't know if this is a precursor to something, if it can be fixed, or just a glitch to live with.

Thanks

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Old 10-03-2017, 11:54 AM   #2
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

I myself would replace the cluster....or if very handy, others may chime in with tips to rebuild the tach cluster.

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Old 10-03-2017, 12:04 PM   #3
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

I uploaded video of tach returning to zero, but can't post link : (

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Old 10-03-2017, 12:40 PM   #4
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Does the engine rev all the way up and attempt to match the tach numbers? Since the PCM has programming to limit rpm to 6750 to prevent engine damage, it's not likely for the engine to rev beyond 6750 at speed. You'd hear it.

1-A tach isn't necessary to automatics. It has nothing to do with actual shifts when its role is to let drivers see engine rpm. A tach is just a distraction for drivers using automatics. The PCM determines all shifts based on sensors from the engine and xmission. My guess is drivers demanded tachs in their automatics from jealousy of manual drivers. At one time all cars and trucks never had tachs: automatics shifted fine while manuals required drivers to shift based on engine sounds as rpm went up.

A car with a manual xmission can be shifted without a tach as was done years ago and still done with most experienced drivers in cars with tachs. It's only necessary for beginners to learn about shifting gears and track racers needing to know when to shift. Even Formula-1 drivers don't have a tach - they're using led bar graphs for a visual indication but rely on engine sounds to know when to shift.

From the service manual;

The tachometer is controlled by the powertrain control module (PCM) using a pulse width modulated (PWM) signal based on the signal received from the crankshaft position (CKP) sensor. The PWM signal to the cluster is passed through an RC filter and sent to the tachometer driver integrated circuit (IC). The high-to-low edge of the PWM signal triggers the IC to provide a well defined pulse of current to the coil of the tachometer. the engine completes one revolution for every six pulses received from the PCM. The PCM monitors circuit 121 for an open, short to ground or short to voltage and will set DTC P1693 if certain parameters are met.


2-Type the link as text, not added as a link so it appears as text.

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Old 10-03-2017, 06:44 PM   #5
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Thanks for responding.

No, the engine acts normally.
The tach incrementally goes up to 8000 and beyond.

I don't really need the tach, but since it is there would be nice to have it work properly.

If as your excerpt from the service manual says, the tach is controlled by PCM with signals sent by crankshaft sensor, I wonder if this issue may be an indication of problems or future problems with either of those.

Or as someone else said it is a problem with the tach itself?

If you send me your email, I can send link to video of it going from 8000 back to zero upon start up.

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Old 10-03-2017, 06:47 PM   #6
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Video link

lahabraheights.net/SL2_tach.MOV

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Old 10-03-2017, 06:54 PM   #7
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

I don't know why, but it still won't load after 8 minutes.....how many MEG is this video anyway?

EDIT:
Ok, speedtest.net showed 1.3mbps UPload, only 2.15mbps Download, Ping 40ms. That is the story of country living..and on a microwave (tower to Motorola square transceiver) link setup.

After watching the video, I still would replace the cluster.

Last edited by toggenburg; 10-03-2017 at 06:59 PM..

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Old 10-03-2017, 07:47 PM   #8
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Since the engine runs fine and shifts are normal, the problem points to the tach. The best you can do may be disconnecting and reconnecting the instrument panel. Exercising the electrical connections can sometimes correct strange problems. Worse case scenario is the IC chip is intermittent and causing the tach needle to wander around incorrectly. I don't know where the IC chip is located but examining for cold solder joints may reveal something. Replacing the instrument cluster is your last option. Used ones from junk yards.
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Old 10-04-2017, 12:32 PM   #9
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Thank you for the assistance.

I'll try unplug and replug.

Though the tach doesn't wander randomly.

It repeats the same scenario going up to over 8000, and then back to zero and acting correctly (for a time) upon restart.

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Old 10-04-2017, 01:08 PM   #10
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Whether its random or not is semantics. The tach is displaying glitches. From your descriptions, this seems to point to the electronics acting sluggishly as if something is delaying the normal power on movement of the tach needle with electronics configured to normalize gauge displays. If you're familiar with dramatic effects, some ads show gauge needles moving from zero to full scale as a WOW factor but of no use in real time operation. These may be described as spikes in early gauge electronics, filtered out with improved electronics to stop needle deflections and allow a linear motion to convey actual interpretation. In this case, an engine not running should show zero rpm and tach needle move in direct proportion to engine rpm. Since the crank sensor detects engine rpm, this signal and ic chip converts varying pulse width signals into a drive voltage to move the tach needle.

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Old 10-05-2017, 05:14 PM   #11
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Default Re: Tach (not engine) goes to 8000 rpm.Back to zero on restart

Thank you for the explanation

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